1. Field of the Invention
The present invention relates to a casing in which a gas container is to be accommodated, referred to as a cylinder cabinet. More specifically, the present invention relates to an apparatus for regulating the flow rate of an air current flowing through an exhaust duct of such a cylinder cabinet.
2. Description of the Prior Art
A special material gas for use in the manufacture of semiconductors is usually used as stored in a portable gas container which is called a gas bomb or gas cylinder. As to such special material gases for use in the manufacture of semiconductors, there are many dangerous gases having combustibility (explosiveness), toxicity, and corrosiveness, and hence they must be used with thoughtful consideration of safety aspects. Accordingly, a gas container filled with a dangerous special material gas has been hitherto accommodated in a casing, which is referred to as a cylinder cabinet, thereby coping with an unforeseen gas leakage.
The cylinder cabinet itself functions to prevent a gas from spreading to the atmosphere, the inside of which is normally ventilated so that a leaked gas can be diluted and safely exhausted therefrom. Accordingly, a cylinder cabinet generally has an air in-take port formed in its lower portion and an exhaust duct provided in its upper portion. Further, this exhaust duct is connected with a main exhaust duct disposed on a building. On the outlet end of said main exhaust duct is fixed an air exhaust fan. If said exhaust fan is driven, the in side of the main exhaust duct will draw air in, whereby air is taken from the air in-take port into the cylinder cabinet and caused to flow through the main exhaust duct by way of the exhaust duct.
Although one is obligated to carry out normal exhaust procedures in a cylinder cabinet in accordance with the relevant laws, it is very important from a safety standpoint that such normal exhaust is carried out at a proper ventilation ratio or ventilation volume depending on the kind of gas used. In the case of a combustible gas such as monosilane, for instance, it is necessary that the ventilation be carried out by introducing air in an amount equal to 10 times the volume of the cylinder cabinet every minute. Further, in the case of a toxic gas, for the purpose of preventing such a gas from leaking from a cylinder cabinet to the atmosphere, ventilation will be carried out so that the flow velocity of an air current at the air in-take port is in the range of from 0.5.about.1 m/sec.
Since the amount of ventilation must be changed depending on the kind of gas to be handled, as mentioned above, it has been hitherto devised that a damper for the flow rate regulation is generally provided in an exhaust duct of the cylinder cabinet.
In general, the exhaust ducts of plural cylinder cabinets are connected to a main exhaust duct. When the exhaust air quantity of, for example, one cylinder cabinet is changed, the pressure of said main exhaust duct varies so that the exhaust air quantity of the other cylinder cabinets should be also changed. Therefore, one approach would be to provide, in a cylinder cabinet, an exhaust sensor for detecting a deviation in the exhaust air quantity from its management or expected value when said deviation takes place. In order to detect this exhaust deviation or abnormality, it is preferable that the air velocity in a duct be measured and the deviation of said air velocity from its management value, which is determined in each of the cylinder cabinets for the kinds of gases accommodated therein, be ascertained. However, there has been hitherto adopted, for economic and technical reasons, a method of estimating an exhaust abnormality indirectly by detecting a pressure difference between the inner and outer pressures of a duct by a fine differential pressure sensor, not by an air velocity sensor. In the prior art, accordingly, a portable air velocity sensor has been separately brought in at every stage where some abnormality in the differential pressure takes place, thereby measuring the air velocity in the duct, and then regulating the damper on the basis of this air velocity data so that a desired exhaust quantity can always be maintained.
In the prior art, a damper provided in an exhaust duct is generally of the butterfly type (which involves rotating a disc to open or close a passage). In the case of a butterfly damper, it is necessary, in order to measure a flow velocity accurately, that a current meter be disposed a distance of ten times the inner diameter of the exhaust duct from the damper. However, the entire length of an exhaust duct in itself is generally shorter then such a distance as it may be less than ten times as large as the inner diameter thereof. Therefore, a current meter can not be disposed at a position sufficiently separate from the dampers. As a result, the precision of the measurement is low and the exhaust air quantity is apt to be unstable.
It is therefore an object of the present invention to provide a flow rate regulation apparatus for an exhaust duct, in which the aforementioned problems can be solved.